This invention relates to photoflash lamps and, more particularly, to flashlamps of the type containing a primer bridge, or the like, ignited by a high-voltage pulse.
Such flashlamps typically comprise a tubular glass envelope constricted and tipped off at one end and closed at the other end by a press seal. A pair of lead-in wires pass through the glass press and terminate in an ignition structure including a glass bead, one or more glass sleeves, or a glass reservoir of some type. A mass of primer material contained on the bead, sleeve or reservoir bridges across and contacts the ends of the lead-in wires. Also disposed within the lamp envelope is a quantity of filamentary metallic combustible, such as shredded zirconium or hafnium foil, and a combustion-supporting gas, such as oxygen, at an initial fill pressure of several atmospheres.
Lamp functioning is initiated by application of a high-voltage pulse (e.g. several hundred to several thousand volts as, for example, from a piezoelectric crystal), across the lamp lead-in wires. The mass of primer within the lamp then breaks down electrically and ignites; its deflagration, in turn, ignites the shredded combustible which burns actinically.
Several different constructions for high-voltage flashlamps have been described in the prior art. The following U.S. Pat. Nos. are examples: 2,718,771; 2,768,517; 2,771,765; 2,868,003; 3,000,200; 3,312,085; 3,501,254; 3,556,699; 3,602,619; 3,627,459; 3,685,947; 3,721,515; 3,823,994; 3,873,260; 3,873,261; 3,884,615; 3,959,860; 4,008,040; 4,059,388; 4,059,389; and 4,097,220. All of these constructions have either been difficult to fabricate, contained extra and costly glass components, or suffered from shred-caused preflash short circuits. Some of the referenced constructions are not adaptable to miniaturization and use in multilamp flash devices of modern design. Many require the use of intricate, tiny glass parts that are very expensive, difficult to feed, and to orient and slip over the lead-in wires in automated, high-speed lamp-making machinery. Reliable automated primer application would not be feasible with some of the designs. Other designs would so vary in firing voltage from one lamp to another that reliable operation could not be obtained with the voltage and energy levels available from miniaturized piezoelectric sources that would fit in the present small cameras. Some of the constructions fail to recognize the problem of shred shorting or shred interference with ignition.
In contrast to the above-mentioned prior art lamp constructions, U.S. Pat. No. 4,059,389 describes a beadless ignition structure with fritcoated inner leads. Although this structure represents a significant improvement in flash reliability and manufacturing simplicity over the prior art, there are problem areas that may arise. For example, if the primer is bridged from lead to lead, expansion differentials from heating to cooling during manufacture sometimes cause the primer bridge to crack open sufficiently so that the high voltage pulse will not jump the gap to flash the lamp. If the lamp is made with separate primer-coated leads, it is necessary that the combustible shreds within the lamp contact both primer-coated lead ends to complete the circuit path. In actual practice, the combustible distribution may occasionally be wadded and located such that it does not make good contact with the primer-coated lead ends and, thus, result in a lamp that fails to flash.